tcmu: release blocks for partially setup cmds

If we cannot setup a cmd because we run out of ring space
or global pages release the blocks before sleeping. This
prevents a deadlock where dev0 has waiting_blocks set and
needs N blocks, but dev1 to devX have each allocated N / X blocks
and also hit the global block limit so they went to sleep.

find_free_blocks is not able to take the sleeping dev's
blocks becaause their waiting_blocks is set and even
if it was not the block returned by find_last_bit could equal
dbi_max. The latter will probably never happen because
DATA_BLOCK_BITS is so high but in the next patches
DATA_BLOCK_BITS and TCMU_GLOBAL_MAX_BLOCKS will be settable so
it might be lower and could happen.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_user.c

@@ -807,6 +807,13 @@ tcmu_queue_cmd_ring(struct tcmu_cmd *tcmu_cmd)
 		int ret;
 		DEFINE_WAIT(__wait);
 
+		/*
+		 * Don't leave commands partially setup because the unmap
+		 * thread might need the blocks to make forward progress.
+		 */
+		tcmu_cmd_free_data(tcmu_cmd, tcmu_cmd->dbi_cur);
+		tcmu_cmd_reset_dbi_cur(tcmu_cmd);
+
 		prepare_to_wait(&udev->wait_cmdr, &__wait, TASK_INTERRUPTIBLE);
 
 		pr_debug("sleeping for ring space\n");